Recombination between repeated sequences is an important genetic interaction in both maintaining and generating sequence diversity. A study of the regulation of recombination between repeated sequences in Saccharomyces cerevisiae is proposed. Regulation will be studied from a genetic and molecular approach. The proposed projects are: (1) A genetic study of repeat recombination in different mutant backgrounds. The types of recombination events that are increased in the hyper-recombination (hpr) mutants will be examined. The types of events that occur in different radiation sensitive (rad) mutants will also be characterized. (2) wild type alleles of hprl, hprll, and hpr12 will be cloned. The nucleotide and protein sequence will be determined. The clones will be used for studies of mRNA expression of the HPR genes. Null alleles and conditional alleles will be constructed in vitro and the in vivo phenotypes determined. Fusion constructions will be made to make fusion proteins for antibody preparation. The antibodies will be used to identify the protein product of the gene. (3) Overexpression of mutant forms of yeast topoisomerase II in yeast will be used to examine the role of topoisomerase II in recombination.